This invention relates generally to inverters, and more particularly, to an improved third harmonic auxiliary commutated inverter having selectable commutation capacitance as a function of inverter load current, thereby providing improved inverter operation as described in my paper entitled "Characteristics of a Current-Fed Inverter with Commutation Applied Through a Neutral Load Point" published in the Conference Record of the 1978 IEEE/IAS Annual Meeting held in Toronto, Ontario, Canada during October of 1978.
Inverters are commonly used in many industrial applications where it is desired to convert potential of one frequency to another frequency, such as is required when a synchronous or induction machine is to be excited from a DC source. The simplest of all types of inverters is the three phase third harmonic auxiliary commutated inverter which can be employed to convert direct current to three phase alternating current to power three phase machines. The three phase third harmonic auxiliary commutated inverter includes three pairs of solid state switching devices, typically thyristors, with the thyristors of each thyristor pair being coupled in series aiding fashion, and each of the thyristor pairs coupled across the DC source. The junction between thyristors of each thyristor pair is coupled to a respective machine phase. A pair of auxiliary thyristors are coupled in series aiding fashion across the DC source and are coupled to the machine neutral by a commutating capacitance.
When the third harmonic auxiliary commutated inverter is employed in load commutated applications, such as in a synchronous machine drive system where the synchronous machine field is over-excited, the thyristors of the three main pairs of inverter thyristors are gated into conduction in a pre-determined sequence to supply the machine with three phase alternating current. Machine back EMF serves to commutate each of the inverter main thyristors once the machine reaches about 40% of operating speed, thus greatly reducing required inverter reactive power. During intervals when machine back EMF is of a magnitude insufficient to commutate a then-conductive main thyristor, a respective one of the pair of auxiliary thyristors is rendered conductive to commutate a then-conductive main thyristor by coupling the commutation capacitance in series with a respective machine phase across the then-conductive main thyristor.
The third harmonic auxiliary commutated inverter is also well suited for non-load commutated applications, such as induction machine drive systems. When employed in such applications, the inverter main thyristors are commutated during inverter operation by rendering an appropriate one of the pair of auxiliary inverter thyristors conductive.
The basic limitation of the third harmonic auxiliary impulse commutated inverter is that during conditions of high machine speed and low current, commutation of an inverter main thyristor may require more than 1/6 f seconds, where f is the frequency of thyristor condution, resulting in both of the auxiliary thyristors becoming simultaneously conductive, causing a short circuit across the DC supply. Since auxiliary inverter commutation time is a function of load current, load inductance and commutation capacitance, the magnitude of the commutation capacitance largely determines the maximum inverter operating frequency, thus limiting inverter operating range.
In contrast, the present invention concerns a third harmonic auxiliary impulse commutated inverter having selectable commutation capacitance as a function of load current to provide increased inverter operating range, and reduced likelihood of inverter solid state switching device over-voltage and inverter commutation failure.